1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a belt fixing device that fixes unfixed toner on a recording sheet through heating and applying pressure, and in particular, to an image forming apparatus wherein a separation performance of a recording sheet after having passed through a nip portion of the belt fixing device is improved.
2. Background of the Invention
In image forming apparatuses of an electrophotographic system such as a copying machine, a printer, a facsimile machine and a multifunctional machine equipped with various functions of the aforesaid equipment, a latent image corresponding to a document is formed on a photoconductor, and the latent image is given toner to be visualized, and a visualized toner image is transferred onto a recording paper, and then, the toner image transferred onto the recording paper is fixed and then, is ejected.
Further, when forming a color image, latent images respectively for Y, M, C and K are formed respectively on four photoconductor drums, and after transferring primarily visualized toner images respectively for four colors onto an intermediate transfer member composed of an endless belt, a toner image is transferred secondarily on the recording paper, and the toner image thus transferred onto the recording paper is fixed, and ejected.
AS a fixing device for fixing a toner image, there is available a heat roller fixing device wherein the recording paper onto which a toner image is transferred is heated and pressed in a nip portion formed by a heat roller having therein a halogen heater and a pressure roller that presses the heat roller, while the recording paper is interposed between the heat roller and the pressure roller to be conveyed, and the fixing device of this kind is widely used because of its simple structure.
There is further available a fixing device of a belt fixing type wherein an endless fixing belt is trained about a heat roller having therein a halogen heater and about a fixing roller, a pressure roller that presses the fixing roller through the fixing belt is provided, and the recording paper onto which a toner image has been transferred is heated and pressed in a nip portion formed by the fixing belt and the pressure roller while the recording paper is interposed between the fixing belt and the pressure roller to be conveyed. The fixing device of this kind has advantages that the time required for a warm-up is shortened because heat capacities of the fixing belt and the heat roller are small, which results in energy saving.
In the fixing device, however, toner of a toner image on the recording paper is heated when the recording paper passes through the nip portion, and the toner acts as adhesives, thus, the recording sheet that has passed through the nip portion is not separated to be wound round the fixing belt while sticking to the surface of the fixing belt, resulting in a fear of occurrence of a jam. When a sheet of paper having light weight (thin paper) is used, especially when coated paper for printing having light weight is used, in particular, separation performance is further reduced.
Further, in the case of a full-color image forming apparatus, it is desirable that images are glossy, and for this reason, it is considered that a nip width is broadened to make the heating time to be longer. For broadening the nip width, a rubber layer of the fixing roller may be thickened, or a diameter of the fixing roller may be made greater.
When using a heat roller fixing device, it is difficult to thicken the rubber layer having a low coefficient of thermal conductivity, because the fixing roller has therein a heat source such as a halogen heater. In addition, if a diameter of the fixing roller is made to be greater, separation performance of recording paper is lowered.
On the other hand, in the belt fixing device, the rubber layer can be thickened because the fixing roller has no heat source, and it is possible to broaden the nip width even when a diameter of the fixing roller is small. Therefore, the belt fixing device is more advantageous than the heat roller fixing device on the point of separation functions of recording paper.
In the case of a high-speed apparatus in particular, the belt fixing device in which a rubber layer can be thickened is advantageous.
However, it is still difficult to separate completely the recording paper having passed through the nip portion, even when the belt fixing device is used.
Accordingly, there is available a fixing device wherein a separation claw is arranged to be in contact with a fixing belt at the downstream side in the conveyance direction of recording paper in the fixing device, so that the recording paper may be separated from the fixing belt. In this structure, however, there is a fear that the fixing belt is worn away to be deteriorated, because the separation claw is constantly in contact with the fixing belt.
There are further disclosed many Patent Documents aiming an improvement of separation functions of recording paper in a fixing device.
For example, there is a belt fixing device wherein a fixing belt is trained about a heat roller, a fixing roller and a fixed member having a small radius of curvature, and the fixed member is in pressure contact with a pressure roller through the fixing belt from the inside of the fixing belt. In this fixing device, separation functions of recording paper are improved without providing a separation claw, because a radius of curvature of the fixing belt is made to be smaller by the fixed member having a small radius of curvature (see Patent Document 1).
There is further a belt fixing device wherein a plurality of separation rotors each having plural claws on its circumference and being driven to rotate are provided at an exit side of a nip portion, and recording paper is separated by the separation rotor from a surface of a fixing belt (see Patent Document 2).
There is still further a belt fixing device wherein a separation member composed of a stainless steel plate is arranged at an exit side of a nip portion on a non-contact with a fixing belt basis, and the separation member prevents recording paper from winding round the fixing belt(see Patent Document 3).
There is furthermore disclosed a belt fixing device wherein a curved portion having large curvature is formed between a nip portion and a pressure contact portion of an auxiliary roller when the auxiliary roller having a small diameter comes in pressure contact with a fixing roller through a fixing belt at an exit side of the nip portion, whereby, recording paper having passed through the nip portion is separated from the fixing belt at the curved portion (see Patent Document 4).
(Patent Document 1) Unexamined Japanese Patent Application Publication No. 2003-5566
(Patent Document 2) Unexamined Japanese Patent Application Publication No. 2003-122172
(Patent Document 3) Unexamined Japanese Patent Application Publication No. 2003-330309
(Patent Document 4) Unexamined Japanese Patent Application Publication No. 2003-330310
However, technologies of Patent Documents mentioned above have the following problems.
In the Patent Document 1, a fixing belt is trained about a heat roller, a fixing roller and a fixed member having a small radius of curvature, and the fixed member is in pressure contact with a pressure roller through the fixing belt from the inside of the fixing belt. Therefore, the reverse side of the fixing belt is rubbed constantly by the fixed member that has a small radius and does not rotate, which results in a problem that the fixing belt tends to be worn away, and sufficient durability cannot be expected.
In the Patent Document 2, there is a problem that an image that has been fixed tends to be scratched by a claw provided on a separation rotor.
In the Patent Document 3, there is a problem that an image that has been fixed tends to be scratched by an edge of a separation member.
In the Patent Document 4, there is a problem that an uneven gloss tends to be generated because close adhesion between a toner image on recording paper and a fixing belt is unstable and irregular, on the downstream side of the nip portion.